1. Field of the Invention
The present invention relates to thermoelectric conversion materials used for a thermoelectric conversion element, or the like.
2. Description of the Related Art
High conversion efficiency from the thermal energy to the electric energy is required to the thermoelectric conversion materials used for a thermoelectric conversion element. That is, the following performances are required. (1) A voltage generated at the time of providing a temperature difference is preferably large so that a high thermoelectric power per the temperature difference 1K is required. (2) In the case the electric resistance is large, since the energy is lost by the Joule's heat at the time an electric current is supplied, a smaller electric resistance is preferable. (3) Since the thermal energy to be converted to the electric energy is escaped as the heat due to the thermal conduction, a smaller thermal conductivity is preferable. From these facts, the characteristics of the thermoelectric conversion materials can be dominated by the value represented by the below formula (i) referred to as a performance index Z.Z=S2·σ/k  (i)
Here, S is the thermoelectric power, σ is the conductivity, and k is the thermal conductivity. A material having a larger performance index Z can provide more excellent thermoelectric conversion materials.
Recently, as an attempt for improving the performance of the thermoelectric conversion materials, dispersion of fine particles in the thermoelectric conversion materials has been studied. In the case a material having a high mobility such as a skutterudite compound and a SiGe is used as the thermoelectric conversion materials, the high conductivity σ of these materials contributes to the performance improvement. On the other hand, due to the high thermal conductivity k thereof, the performance index Z has not been made larger. However, by dispersing the fine particles in such thermoelectric conversion materials, since the phonons are scattered by the fine particles, the thermal conductivity k can be made smaller so as to improve the performance.
For example, the thermoelectric conversion materials having FeSb2 fine particles introduced in the matrix of a skutterudite compound to 40% by the mole ratio by the mechanical alloying process are proposed in J. Applied Physics, 88, p. 3484-3489 (2000). It is disclosed that the size of the FeSb2 fine particles is of the submicron order, and the performance index in a range of 600K to 800K of the substance with the FeSb2 fine particles introduced is larger than the substance without the introduction of the FeSb2 fine particles. However, the performance improvement of the introduction of the FeSb2 fine particles is not shown at 500K or lower.
Moreover, according to Materials Science and Engineering, B41, p. 280-288 (1996), a 0.7 nm particle size fullerite (a mixture of C60: 90% and C70: 10%) is introduced into a SiGe alloy (Ge: 20 atm %) to 1 wt % by the mechanical alloying process. However, the fullerite dispersing property is not disclosed and the performance improvement is not mentioned. The main cause why the performance is not improved is considered to be the reduction of the conductivity σ accompanied by the reduction of the thermal conductivity k by the introduction of the fine particles.
Furthermore, the 18th International Conference on Thermoelectrics, p. 280-284 (1999) proposes a method for improving the performance by a PbTe/PbSeTe based quantum dot super lattice. The quantum dot super lattice is produced by alternately laminating a PbTe and a PbSeTe (composition mole ratio Pb:Se:Te=1.00:0.98:0.02, hereinafter it is omitted) by the molecular beam epitaxy method, utilizing the granular change of a film of the PbSeTe formed on the PbTe from a thin film immediately after the film formation by the lattice unconformity with the PbTe according to the time passage. By further forming a film of the PbTe so as to cover the PbSeTe particles having had the granular change on the PbTe film and again forming a film of the PbSeTe, a quantum dot super lattice with the PbSeTe fine particles (quantum dots) regularly dispersed in the PbTe can be obtained. The size of the PbSeTe fine particles was 30 nm or less. In a range of 300K to 500K, the system with the PbSeTe fine particle introduction in the PbTe showed the performance index larger than the system without the introduction by as much as 7 times to 13 times. This is considered that not only the above-mentioned phonon scattering effect but also the quantum effect contribute to the performance improvement. This theory is described in detail in Phys. Rev. B, 47, p. 16631 (1993). In general, the size having the quantum effect appearance is 100 nm or less.
However, since the quantum dot super lattice is produced by alternately laminating the PbTe and the PbSeTe in a vacuum system by the molecular beam epitaxy method as mentioned above, a problem is involved in that much labor is required. Moreover, the thickness of each layer is about 10 nm. However, in the case it is used as the thermoelectric conversion materials, a 10 nm or more thickness is needed, and thus at least 500 each layers need to be laminated, and thus it is not suitable for the mass production.
On the other hand, although it is not a system with the fine particles dispersed in a thermoelectric conversion material, Nanostructured Films and Coatings, p. 149-156 (2000) proposes a method of covering SiO2 fine particles with a skutterudite compound. This is a method of coating a gold by sputtering on SiO2 fine particles having a 300 nm particle size with 10 to 60 nm thickness, and furthermore, coating a CoSb3 as one kind of the skutterudite compounds thereon. The CoSb3 is formed by reduction firing after coating a precursor thereof on the gold. However, according to this method, the SiO2 fine particles are used merely as a supporting substance without describing the phonon scattering effect by the SiO2 fine particles. Moreover, since a reducing gas is used at the time of reducing the precursor of the CoSb3, a problem is involved in that the precursor may not be reduced sufficiently to the inside.